I think the big thing is trust in the market. Consumers tend to be conservative in which parts of the market they trust. They don't want to pay for something if they have no idea if they're going to get their money's worth. With existing markets, or any market that lots of people are using, people trust that it's okay so they join. With something new, are you going to pay money to access something if you don't know if it's any good? Moving towards free+ads is always easier than towards payment up front.

That said, Netflix is incredibly successful with something new, but that also took years. Netflix worked hard on the reputation that they had the best, most interesting new TV shows that you couldn't get anywhere else. But it takes years to overcome that inertia.